OPTICAL INVESTIGATIONS OF THE EFFECTS OF STRATIFICATION ON HOMOGENEOUS CHARGE COMPRESSION IGNITION COMBUSTION
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Ghandhi, Jaal B.
- Jul 08, 2009
- The effects that thermal and compositional stratification have on homogeneous
charge compression ignition (HCCI) combustion were studied using an optically
accessible internal combustion engine. A stratified flowfield was developed in the engine
by feeding each intake valve of the four-valve engine with independent intake systems.
Planar laser-induced fluorescence (PLIF) of 3-pentanone that was introduced
through only one intake valve indicated significant mixing between the two intake
streams. A number of different intake-flow modifying devices were used in an attempt to
maximize the amount of bulk stratification maintained throughout compression, but only
when using top- and inside-directing intake baffles were significant improvements over a
simple, straight-runner system observed. The bulk stratification maintained throughout
compression, measured as the average deviation of the mean fluorescence profile from
the mean homogeneous fluorescence profile, increased by 36% when using the topdirecting
baffles and by 30% when using the inside-directing baffles compared to when
using the same runner with no baffles.
The combination of cylinder pressure, engine-out emissions, and high-speed
chemiluminescence measurements were used to evaluate the effects that stratification had
on HCCI combustion. The cylinder pressure and emissions data showed little-to-no
difference when comparing the combustion under homogeneous operation to combustion
under stratified operation at a constant location of peak pressure. Large differences,
however, could be observed in the spatial progression of the HCCI combustion.
Qualitative observations of the manner in which the combustion proceeded indicated that
�60 K temperature stratification, �25% fuel concentration stratification, and �7 air-fuel
ratio stratification all similarly affected the combustion progression.
A dual-tracer PLIF temperature imaging technique was calibrated in situ and
applied under motored and fired engine operation. Initial experiments under motored
engine operation showed that the dual-tracer PLIF technique, with 3-pentanone and
triethylamine as the tracers, achieved sufficient temperature precision to measure singleshot
temperature variations of �2.3 K (�1?) with an intensified camera or �1.4 K (�1?)
with an unintensified camera. Mean temperature profiles acquired with thermally
stratified intake conditions showed a 5 K gradient across the combustion chamber. When
applied under fired operation, the high in-cylinder temperatures resulted in low
fluorescence signals and limited the ability to precisely measure temperature variations
resulting from introduced thermal stratifications.
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